I can't use my desktop resolution (1920x1200) as its too slow, so I thought I'd choose a lower resolution but at the same aspect ratio (16:10). But when I choose a lower res, the game is not "filling" the screen. It shows blank area above and below the game. Why doesn't the game "expand" to fill the spaces? Other games, like Dragon Age, have no problem filling the screen at a lower resolution.

I then though I might try windowed mode.

The first time I set it up the game crashed saying it needed 1.96GB or something (my PC has 4GB RAM). Then when I try to start it up again it won't start at all giving an error: Couldn't create D3D device.
Wondering what's happening there? I didn't have this issue before. I was able to switch without a problem between fullscreen and windowed mode.